Metalic nanowire and process for producing the same

ABSTRACT

A nanowire comprising only metal having an average length of 1 μm or more which could not be produced in the prior art, and a method of manufacturing this wire. 
     This invention provides a method of manufacturing a metal nanowire, which comprises the step of reducing a nanofiber comprising a metal complex peptide lipid formed from the two-headed peptide lipid represented by the general formula (I): 
                 
 
in which Val is a valine residue, m is 1-3 and n is 6-18, and a metal ion, using 5-10 equivalents of a reducing agent relative to the two-headed peptide lipid. It further provides a metal nanowire having an average diameter of 10-20 nm and average length of 1 μm or more. It is preferred that the metal is copper.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a nanowire comprising only metal, and to amethod of manufacturing same. More specifically, it relates to a metalnanowire of average length at least 1 μm, and its method of manufacture.This metal nanowire can be used as a nanoelectron part or nanomagneticmaterial in industrial fields such as electronics/information.

Prior Art

In the prior art, a method is known for manufacturing a copper cylinderstructure wherein an organic solution containing water in which anorganic aerogel-forming material complexed with copper (II) ion isreduced by hydrazine (e.g., M. P. Pileni et. al., Lamgmuir 1998, 14,7359-7363). However, the cylindrical structure obtained by this methodranges at most from several tens to several hundred nm, and it was notpossible to produce a long fiber structure.

It is disclosed in Japanese Patent No. 3012932 that, when an aqueoussolution containing a two-headed peptide lipid as alkali metal salt isleft to stand in steam saturated with a 1-5 wt % acidic solution,microfine fibers are obtained due to the one-dimensional crystal growthor self-deposition of this peptide lipid. However, the fibers obtainedby this method comprised only organic substances.

On the other hand, the inventors have already reported that a hybridnanofiber is obtained by adding a metal ion to the alkali metal salt ofa two-headed lipid (“Manufacture of Organic/Inorganic HybridNano-Structures by Self-deposition”, in No. 49 Polymer Symposium, onSep. 29, 2000). This fiber is a hybrid of an organic substance and ametal, and was not a fiber comprising only metal.

Problems to be Solved by the Invention

It is therefore an object of this invention, by making use of thesefacts, to provide a nanowire comprising only metal having an averagelength of 1 μm or more which could not be produced in the prior art, anda method of manufacturing this wire.

Means to Solve the Problems

The inventor, as a result of intensive studies to develop a simplemethod of manufacturing a metal nanowire having an average length of at1 μm or more, discovered that it was possible to manufacture such ananowire comprising only metal and having a length of 1 μm or more whichwas not available in the prior art, by chemically reducing a hybridnanofiber produced by adding a metal ion to a two-headed peptide lipidusing 5-10 equivalents of a reducing agent in water.

Specifically, it is an object of this invention to provide a method ofmanufacturing a metal nanowire, which comprises the step of reducing ananofiber comprising a metal complex peptide lipid formed from thetwo-headed peptide lipid represented by the general formula (I):

in which Val is a valine residue, m is 1-3 and n is 6-18, and a metalion, using 5-10 equivalents of a reducing agent relative to thetwo-headed peptide lipid.

According to this method, a nanofiber for which the initialconcentration of the metal complex peptide lipid is 0.1-1 mmoles/litermay be reduced in aqueous solution using copper (II) ion as the metalion and sodium borohydride as the reducing agent, or a nanofiber forwhich the initial concentration of the metal complex peptide lipid is10-15 mmoles/liter may be reduced in aqueous solution using copper (II)ion as the metal ion and hydrazine as the reducing agent. This initialconcentration means the concentration of the metal complex peptide lipidin aqueous solution prior to adding the reducing agent.

It is a further object of this invention to provide a metal nanowirehaving an average diameter of 10 to 20 nm and an average length of 1 μmor more. It is preferred that this metal is copper.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmission electron micrograph of a copper nanowireobtained by Example 1.

FIG. 2 is a diagram which traces the transmittance electron micrographof the copper nanowire obtained in Example 1.

DETAILED DESCRIPTION OF THE INVENTION

The method of manufacturing the metal nanowire of this inventioncomprises the steps of making a colloidal dispersion of nanofibers byadding a metal ion to an aqueous solution containing the two-headedpeptide lipid represented by the following general formula (I)

in which Val, m and n are identical to the above, and adding a reducingagent.

The two-headed peptide lipid having a structure represented by thefollowing general formula (I):

in which Val, m and n are identical to the above, is formed by joiningthe oligomer of an optically active L-valine residue or D-valine residueto a long chain dicarboxylic acid via an amide bond, having the Cterminal of the oligopeptide chain at both ends.

The valine residue forming the oligopeptide chain is represented by thefollowing formula:

and its optical activity must be entirely D or L.

If a different optically active substance is contained therein, ananofiber is not formed and a particulate amorphous solid is formedinstead. m is 1-3. If m is 4 or higher, the solubility of the compoundis poorer, and it is difficult to manufacture the nanofiber of thisinvention. Further, n gives the length of the straight chain alkalenegroup, and is 6-18. Examples of this alkalene group are hexalene,heptylene, octalene, nonalene, decylene, undecyline, dodecylene,tetradecylene, hexadecylene and octadecylene. If n is less than 6, it isdifficult to form the nanofiber, and if it is higher than 18, theprecipitates formed in the aqueous medium become amorphous spheres.

When a metal ion is added to the sodium salt of this two-headed lipid inaqueous solution, as a result of self-deposition, a colloidal dispersionis formed. Although there is no particular limitation on conditions suchas temperature, it is desirable to stir the mixture well. Examples ofthis metal ion are Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺, but Cu²⁺ is tobe preferred. Any method may be used to introduce this metal into thereaction liquor, but it is convenient to introduce it as a metal salt.For this purpose, a salt of an inorganic acid or an organic acid may beused.

When the reducing agent is added to this colloidal solution, the metalnanowire is produced. Specifically, as the two-headed lipid is dissolvedin water as the sodium salt by reduction, a nanowire comprising onlymetal obtained. There is no particular limitation on conditions such astemperature, but it is preferable to continue stirring.

There is no particular limitation on the reducing agent, examples beinghydrogen, or relatively unstable hydrogen compounds such as hydrogeniodide, hydrogen sulphide, aluminium lithium hydride and sodiumborohydride, lower oxides or salts of lower oxides such as carbonmonoxide, sulphur dioxide and bisulphates; sulphur compounds such assodium sulphide, sodium polysulphide and ammonium sulphide; metalshaving a high electropositivity such as alkali metals, magnesium,calcium and aluminum, and their amalgams; or organic compounds having alow oxidation state such as aldehydes, sugars, formic acid, oxalic acidand hydrazine, but sodium borohydride or hydrazine are preferred.

The amount of reducing agent is 5-10 equivalents relative to thetwo-headed peptide lipid. When the amount of reducing agent is less than5 equivalents, reduction does not proceed to completion, and when it ishigher than 10 equivalents, reduction proceeds so rapidly that largelumps are formed and the copper nanowire is not formed.

It is preferred to suitably select the concentration of the metalcomplex peptide lipid in the colloidal dispersion when the reducingagent is added, according to the strength or weakness of the reducingagent. If a reducing agent having strong reducing properties is used,the concentration (initial concentration) of the two-headed peptidelipid when the reducing agent is added is preferably low, whereas when areducing agent having weak reducing properties is used, theconcentration (initial concentration) of the two-headed peptide lipidwhen the reducing agent is added is preferably high. For example, whensodium borohydride is used as the reducing agent, the concentration(initial concentration) of of the metal complex peptide lipid mayconveniently be 0.1-1 mmol per litre, and when hydrazine is used as thereducing agent, the concentration (initial concentration) of the metalcomplex peptide lipid may conveniently be 10-15 mmol per litre. If thecolloidal dispersion is too thin, no structure of any kind can beformed, whereas if it is too dense, large lumps are produced and thecopper nanowire cannot be formed.

In this way, when the reducing agent is added while stirring thecolloidal suspension, this solution gradually changes and forms a metalnanowire after several hours. The length of this metal nanowire is anaverage of 1 μm or more, preferably 1 mm or less, more preferably 100 μmor less and still more preferably 1-10 μm. This length naturally varieswith the manufacturing conditions. Also, as seen from the photographs(FIGS. 1 and 2) shown in the following examples, various lengths of thismetal nanowire may be mixed together, but the salient feature is thatthey comprise wires of 1 μm or more, and this length had not beenobserved in the prior art. The long wire may be extracted by any method,or it may be used in admixture with shorter wires. The diameter of thismetal nanowire is an average of 10-20 nm. Nanowires of diameters outsidethis range may also be present depending on the manufacturingconditions, but it is considered that, on average, the diameter lieswithin this range, as seen from the following examples.

This invention will now be described by way of specific examples, but itmust be understood that the invention is not to be construed as beinglimited in anyway thereby.

MANUFACTURING EXAMPLE 1

10.9 g (50.0 mmol) of t-butyloxycarbonyl-L-valine, 19.0 g (50.0 mmol) ofp-toluene sulfonic acid salt and 7.0 ml (50.0 mmol) of triethylaminewere dissolved in 150 ml of dichloromethane, 100 ml of a dichloromethane solution containing 10.5 g (55.0 mmol) of1-ethyl-3-(3-dimethylaminopropyl) carboimido hydrochloride were added at−5 degree C. with stirring, and stirring was continued for 24 hours.This dichloromethane solution was washed twice with each of a 10 wt % ofcitric acid aqueous solution, water, 4 wt % sodium bicarbonate aqueoussolution and water, and the organic layer was dried over anhydroussodium sulfate. The solvent was distilled off completely under reducedpressure to give a colorless, transparent oil oft-butyloxycarbonyl-L-valyl-L-valinebenzylester. This oil was dissolvedin 100 ml of ethyl acetate, 120 ml 4N-hydrochloric acid/ethyl acetatewas added, and the mixture stirred for 4 hours. The solvent wasdistilled off completely under reduced pressure, diethyl ether was addedto wash the white precipitate well, and 13.8 g of a white solid ofL-valyl-L-valinebenzylester hydrochloride was obtained (yield 80%).

0.46 g (2 mmol) of 1,10-decanedicarboxylic acid and 0.674 g (4.4 mol) of1-hydroxybenzotriazole were dissolved in N,N-dimethylformamide, and 10ml of a dichloromethane solution containing 0.90 g (4.4 mmol) of1-ethyl-3-(3-dimethylaminopropyl) carboimido hydrochloride was added at−5 degree C. with stirring. After 1 hour, 10 ml of dichloromethanesolution containing 1.51 g (4.4 mmol) of the aboveL-valyl-L-valinebenzylester hydrochloride followed by 0.62 ml (4.4 mmol)of triethylamine were added, and stirred for 24 hours while graduallyreturning to room temperature. The solvent was completely distilled offunder reduced pressure, and the white precipitate obtained was washed onfilter paper successively with 50 ml of 10 wt % citric acid aqueoussolution, 20 ml water, 50 ml of 4 wt % sodium bicarbonate aqueoussolution and 20 ml water. 0.98 g of N,N′-bis(L-valyl-L-valinebenzylester) decane-1,10-dicarboxamide was obtained asa white solid (yield 0.61%). 0.5 g (0.62 mmol) of this compound wasdissolved in 100 ml dimethylformamide, 0.25 g of 10 wt %palladium/carbon was added as a catalyst, and catalytic hydrogenationwas performed. After 6 hours, the catalyst was filtered off usingcerite, and the solvent was distilled off under reduced pressure toobtain a colorless oil. The oil obtained was crystallized using awater-ethanol mixed solvent to give a white solid. After analysis, thiswhite solid was N,N′-bis (L-valyl-L-valine) decane-1,10-dicarboxamide(corresponds to m=2, n=10 in general formula (1)).

EXAMPLE 1

0.1 mmol of the two-headed peptide lipid obtained in ManufacturingExample 1 was taken in a sample bottle, 100 ml of distilled watercontaining 8.0 mg (0.20 mmol) of sodium hydroxide (2 equivalents) wasadded, and the two-headed peptide lipid was dissolved by applyingultrasonic irradiation (pass type).

This aqueous solution was maintained at room temperature while stirringvigorously over a hot stirrer. When 1 ml of 0.1 mol/liter of copper (II)acetate was added, the solution gradually became cloudy, and a bluecollolidal dispersion was formed. This blue colloidal dispersion wasstirred at room temperature in the atmosphere. When 100 ml (0.5 mmol) of5 mmol/liter of sodium borohydride aqueous solution was added, thesolution immediately turned blackish brown, and after about 6 hours, adark grey filamentous precipitate formed. When this filamentousprecipitate was examined under a transmission electron microscope,spherical structures of diameter several tens-several hundrednanometers, and the formation of a copper nanowire, were observed. FIG.1 and FIG. 2 show transmission electron micrographs of the coppernanowire obtained. As can be seen from these photographs, the averagediameter of this copper nanowire was 10-20 nm and its average length was1-10 μm or more.

EXAMPLE 2

1.0 mmol of the two-headed peptide lipid obtained in ManufacturingExample 1 was taken in a sample bottle, 100 ml of distilled watercontaining 80.0 mg (2.0. mmol) of sodium hydroxide (2 equivalents) wasadded, and the two-headed peptide lipid was dissolved by applyingultrasonic irradiation (pass type).

This aqueous solution was maintained at room temperature while stirringvigorously over a hot stirrer. When 1 ml of 0.1 mol/liter of copper (II)acetate was added, the solution gradually became cloudy, and a bluecollolidal dispersion was formed. This blue colloidal dispersion wasstirred at room temperature in the atmosphere. When 9.2 ml (10 mmol) ofa 35 wt % hydrazine aqueous solution was added, the solution immediatelyturned yellow, and after about 6 hours, a yellow colloidal precipitateformed. When this filamentous precipitate was examined under atransmission electron microscope, the formation of a copper nanowirehaving a length of several—several hundred μm and a diameter of severalnanometers, was observed.

According to this invention, a metal nanowire having an average lengthof 1 μm or more, which could not be produced from a synthetic compounduntil now, can easily be manufactured under the mild conditions of roomtemperature and atmospheric pressure. As the nanowire of this inventioncomprises only metals, it is electrically conducting, and has manifoldindustrial applications, such as in the electronics/information fieldswhich use nanoelectron parts and nanomagnetic materials.

1. A method of manufacturing a metal nanowire, which comprises the stepof reducing a nanofiber comprising a metal complex peptide lipid formedfrom the two-headed peptide lipid represented by the general formula(I):

in which Val is a valine residue, m is 1-3 and n is 6-18, and a metalion, using 5-10 equivalents of a reducing agent relative to thetwo-headed peptide lipid.
 2. The method of manufacturing the metalnanowire as defined in claim 1, wherein a nanofiber, wherein the initialconcentration of the metal complex peptide lipid is 0.1-1 mmoles/liter,is reduced in aqueous solution using copper (II) ion as the metal ionand sodium borohydride as the reducing agent.
 3. The method ofmanufacturing the metal nanowire as defined in claim 1, wherein ananofiber, wherein the initial concentration of the metal complexpeptide lipid is 10-15 mmoles/liter, is reduced in aqueous solutionusing copper (II) ion as the metal ion and hydrazine as the reducingagent.
 4. The metal nanowire having an average diameter of 10 to 20 nmand an average length of 1 μm or more, which is produced by the methodof claim
 1. 5. A metal nanowire having an average diameter of 10 to 20nm and an average length of 1 μm or more, which is produced by themethod of claim
 2. 6. A metal nanowire having an average diameter of 10to 20 nm and an average length of 1 μm or more, which is produced by themethod of claim
 3. 7. A metal nanowire having an average diameter of 10to 20 mn and an average length of 1 micron or more, which is producedusing as a template a nanofiber comprising a metal complex peptideformed from the two-headed peptide lipid represented by the generalformula (I):

in which Val is a valine residue, m is 1-3 and n is 6-18, and a metalion.
 8. The metal nanowire as defined in claim 7, wherein the metal iscopper.